Estimating in situ exposure in the presence of acoustic nonlinearity

Evidence is presented for excess attenuation of pulsed ultrasound due to fi nite amplitude effects in water. Measurements on a modern scanner are used to demonstrate that linear derating can underestimate many exposure quantit ies, including all safety indices apart from the cranial thermal index. Mor e appropriate methods for estimating in situ exposure are reviewed. A prefe rred procedure that requires exposure measurements to be made in water unde r "small signal" conditions is selected. A spectral index is defined that i s proposed as an indicator of finite amplitude effects, where spectral inde x = 0.1 defines the threshold between nonlinear and quasi-linear conditions .